Re: [Qemu-devel] [qemu-s390x] [PATCH] s390x/pci: add common fmb

[Thomas Huth <thuth@redhat.com>]

On 2018-09-29 07:48, Yi Min Zhao wrote:
> 
> 在 2018/9/19 下午3:53, Thomas Huth 写道:
>> On 2018-09-19 09:08, Yi Min Zhao wrote:
[...]
>>>> diff --git a/hw/s390x/s390-pci-bus.h b/hw/s390x/s390-pci-bus.h
>>>> index 1f7f9b5814..fdf13a19c0 100644
>>>> --- a/hw/s390x/s390-pci-bus.h
>>>> +++ b/hw/s390x/s390-pci-bus.h
>>>> @@ -286,6 +286,28 @@ typedef struct S390PCIIOMMUTable {
>>>>        S390PCIIOMMU *iommu[PCI_SLOT_MAX];
>>>>    } S390PCIIOMMUTable;
>>>>    +/* Function Measurement Block */
>>>> +#define DEFAULT_MUI 4000
>>>> +#define UPDATE_U_BIT 0x1ULL
>>>> +#define FMBK_MASK 0xfULL
>>>> +
>>>> +typedef struct ZpciFmbFmt0 {
>>>> +    uint64_t dma_rbytes;
>>>> +    uint64_t dma_wbytes;
>>>> +} ZpciFmbFmt0;
>>>> +
>>>> +typedef struct ZpciFmb {
>>>> +    uint8_t format;
>>>> +    uint8_t fmt_ind[3];
>>>> +    uint32_t sample;
>>>> +    uint64_t last_update;
>>>> +    uint64_t ld_ops;
>>>> +    uint64_t st_ops;
>>>> +    uint64_t stb_ops;
>>>> +    uint64_t rpcit_ops;
>>>> +    ZpciFmbFmt0 fmt0;
>>>> +} QEMU_PACKED __attribute((__aligned__(8))) ZpciFmb;
>> All the fields in this struct are naturally aligned already, so I'd
>> maybe rather drop the QEMU_PACKED and add a
>> QEMU_BUILD_BUG_MSG(sizeof(ZpciFmb) != xx, ...) statement afterwards.
> Currently we only implement FMT0. There're other FMTs to be implemented
> in future.
> So here there would be a union and we can't give a specific size to
> QEMU_BUILD_BUG_MSG.
> Can we use the max size for checking?

I think you could use this to check the beginning of the struct:

QEMU_BUILD_BUG_MSG(offsetof(ZpciFmb, fmt0) != 48, "padding in ZpciFmb");

>>>>    struct S390PCIBusDevice {
>>>>        DeviceState qdev;
>>>>        PCIDevice *pdev;
>>>> @@ -297,6 +319,8 @@ struct S390PCIBusDevice {
>>>>        uint32_t fid;
>>>>        bool fid_defined;
>>>>        uint64_t fmb_addr;
>>>> +    ZpciFmb fmb;
>> ... since you embed it here in another struct which does not have any
>> alignment requirements. This is likely going to cause an error with GCC
>> 8.1, we've had this problem in the past already:
>>
>> https://git.qemu.org/?p=qemu.git;a=commitdiff;h=a6e4385dea94850d7b06b0
> Ah...I didn't test the code with gcc 8+. GCC I used is 7.2.
> It should get the same warining.

Nobody reported the warning in the s390-ccw bios until GCC 8 had been
released, so I assume this is a new warning in GCC 8.

>> Is the __align__(8) required at all? As far as I understand the code,
>> the struct is not living inside the guest memory, is it? So you could
>> simply drop the __align__(8).
>> But if you need it, I think you have to allocate the memory for ZpciFmb
>> separately (and use a "ZpciFmb *fmb" here instead).
> I want to copy the entire structure to the guest memory during updating
> FMB.
> It's not a good idea to do copy for all members multiple times.

Sure, but you're doing the updates through address_space_write(), so as
far as I can see there is currently no need for the
attribute((__aligned__(8))) here (or did I miss something?). Thus I'd
like to suggest to simply remove that attribute here.

 Thomas